This invention relates to a method and means for effecting phase rotation of signals and is especially useful in electronic navigation systems.
A recent development in navigation is a system known as NAVSTAR, utilising a number of earth satellites.
The complete Navstar system is planned to consist of 18 satellites arranged in nearly circular orbits with a radii of 26,600 Km, and an inclination to the earth's equatorial plane 55.degree.. Each satellite transmits two navigation signals, designated L1 and L2 and centred on 1575 and 1228 MHz respectively.
Both signals convey ranging information by means of modulations which are locked in time to atomic standards. The forms of these modulations (which are known as pseudorandom codes because they appear random but are nevertheless well defined) are unique to each satellite.
By measuring the phases of the received codes against a clock in the receiver, together with the Doppler shifts of the radio frequency carriers, a user can calculate the range and range rate to a particular satellite by monitoring four satellites. By decoding data about their motions which are also modulated onto the transmitted signals, the user may solve equations to determine his three-dimensional position and velocity and also apply corrections to his clock, making it conform to satellite time.
Two pseudorandom codes are in fact transmitted by each satellite. The first of these is used to aid acquisition of the satellite signals and to provide coarse navigation, and hence is called the Coarse/Acquisition (C/A) code. The second has a 10-times higher modulation rate which yields the full navigational accuracy of the system, and is designated the Precision (P) code.
A basic Navstar receiver typically contains a low-noise amplifier and down-converter to a convenient IF, followed by one or more code and carrier tracking channels, each capable of tracking the transmissions from any satellite. There are also associated range and range-rate measurement circuits.
The purpose of the code tracking loop is to keep a code generator in the receiver in step with a received pseudorandom sequence, and hence provide information on the range to the satellite being tracked.
One implementation of a Navstar receiver includes amplification of the received r.f. signals and down conversion to i.f. frequencies to produce quadrature signals; analogue-to-digital converters to digitise separately the quadrature signals; local digital code generating means; means for correlating the digitised quadrature signals separately with the same locally generated digital codes; channel processing means to which the outputs of the correlation means are applied, the processing means being arranged to control the code and carrier tracking of the receiver; and correction means responsive to control signals generated in the processing means to effect phase rotation of the baseband signal phasor represented by the quadrature signals to effect Doppler tracking in the receiver loop. Any Doppler shift will cause the phasor to rotate and by removing this rotation the Doppler shift can be tracked. To remove the phase rotation from a signal vector the latter must be multiplied by a counter-rotating unit vector. Attention is directed to our U.S. application Ser. No. 706,310, now U.S. Pat. No. 4,651,154 filed Feb. 27, 1985, which discusses this aspect in greater detail. The above referred to receiver implementation further includes a digital data memory means loaded with data in the form of "look-up" tables incorporating combinations of signal input combinations, means for applying the digitised quadrature signals as partial address signals for the memory means, and means for generating additional address signals for the memory means in response to control signals whereby phase rotation correction is effected by accessing the memory means in accordance with the address signals to produce an output signal for the channel signal processing means.
In our U.S. application Ser. No. 842119 filed May 20, 1986 there is described a method and apparatus for effecting phase rotation which involves a transformation on the digitised I and Q vectors in order to overcome accuracy problems arising because of the compressions and truncation involved in rotating a uniformally arranged square grid of points within a square grid of requantisation boundaries. This can be implemented by incorporating a modification in the phase rotation look-up table. In particular this second Application provides a method of effecting phase rotation of a digital signal by applying said digital signal as partial address signals for a look-up table in a memory together with rotation control signals forming additional address signals whereby the memory output represents the phase rotated signal, each input vector being modified so that points representing digitised input vector values plotted on a rectangular co-ordinate grid are transformed to lie on an alternative co-ordinate system, in particular a circular system, the rotated vector magnitudes being retransformed and requantised to a digital output.
Difficulties have arisen when putting the above referred to phase rotator into practice in certain cases owing to size of the PROM required for the look-up table. The present invention thus seeks to reduce the size of the PROM required for the look-up table.